Quote RCapes:When all soundcards
have clocks in them, what is the purpose of these very expensive, separate clocks?

The clocks in sound cards can't
always be synchronised to some types of external reference, such as video sync, which is
often necessary in audio-visual applications for example.

Also, it is sometimes
more practical to use a master clock to drive everything from a central point, rather than
chain the clock from a soundcard around a complex system.

For most small
digital systems the clock in the soundcard is entirely adequate and can be distributed to
the few units that need it without much problem.

And from a techniacl point of
view, it is always better to run the main A-D from its internal clock. With only one or
two exceptions, running an A-D from an external clock degrades its technical performance,
sometimes quite significantly.

separate clocks are only really useful in complex digital systems where multiple devices
need to be synchronised together... for example, if you have a protools rig, and logic
rig, and a hardware recorder, and some digital FX, and a couple of digital 2 track
recorders (like we do) then you need to get everything sync'ed together from one master
clock, and doing so in parallel form a clock with multiple dedicated outputs is far more
reliably stable than daisy chaining them all .

especially if some of the
clocks use different formats, like Protools does, a master clock capable of generating
all the different formats is essential.